1. [Field of the Invention]
This invention relates to an ultrasonic imaging apparatus in which pulse voltages for exciting transducer elements are controlled in accordance with the scanning mode, or the transducer characteristics.
2. [Description of Prior Art]
In an ultrasonic imaging apparatus, a transducer having a plurality of transducer elements, e.g., 32, 48, 64 and 128 arranged in an array is energized by exciting pulses to transmit ultrasonic pulses toward the interior of a body, thereby receiving ultrasonic echoes reflected therefrom to produce echo signals. The echo signals contain medical information such as bloodflow conditions of the body under examination.
Generally, three typical exciting pulses are utilized in the current ultrasonic imaging apparatus. As the first exciting pulses, a single pulse is repeatedly applied to the transducer elements every rate pulse period. However, amplitudes of the reflected echo signals are gradually decreased, as the ultrasonic echos are reflected from blood cells in the bloodflow. As a result, the signal-to-noise ratio is lowered.
To avoid the above drawback, the burst drive pulses are employed. For example, the burst drive pulses having three wave numbers are produced every rate pulse period. This second exciting method can provide various merits. That is, the signal-to-noise ratio can be improved by limiting the frequency range, compared with the first exciting method. Moreover, other medical information, i.e., flow directions of the bloodflows can be obtained. However, the other problems occur. Since the power of the burst drive pulses is greater than that of the single drive pulse, the average power of the applied ultrasonic pulses increases and thus the power dissipation in the transducer elements also increases. Eventually, the heat radiation of the transducer becomes great, resulting in safety problems for a human body due to high temperatures of the transducer.
When the pulses Doppler method is introduced in the above ultrasonic imaging apparatus so as to detect the bloodflow velocity, the maximum bloodflow velocity is subjected to be limited by the rate frequency. Accordingly, the rate frequency needs to be set so high to detect such a higher bloodflow velocity. As previously described, the higher the rate frequency is set, the greater the average power of the applied ultrasonic pulses becomes and also the higher the temperature of the transducer becomes due to the power dissipation therein.
According to another method to detect such a higher bloodflow velocity, continuous wave (CW) drive pulses are utilized, whose duration is rather longer than that of the single drive pulses. However, the average power of the transmitted ultrasonic pulses and also the temperature of the transducer become higher than those in the first drive pulse.
Another drawback of the conventional imaging apparatus is caused by the scanning modes. In the sector and linear scanning modes, the power dissipation of the transducers is different from each other, since voltages of the drive (exciting) pulses derived from the pulser are constant. That is, in the linear scanning mode, several elements of the transducer are simultaneously excited by the drive pulses, but the remaining transducer elements are in the waiting condition, i.e., no power dissipation. To the contrary, all transducer elements of the sector scanning mode are simultaneously excited, so that if the maximum pulse voltage in the linear scanning mode is applied to the sector type transducer, the temperature of the sector type transducer is necessarily higher than the allowable temperature thereof.
In other words, if the power dissipation, or the temperature increase of both the transducers is maintained constant, the signal-to-noise ratio of the lower temperature transducer (i.e., the linear type transducer) is degraded. Such trade-off conditions cannot be sufficiently solved in the conventional imaging apparatus.
It is therefore an object of the present invention to efficiently drive the transducer elements under the suitable drive voltages in the ultrasonic imaging apparatus.